Satellite positioning systems are typically used to accurately determine the position of users possessing special navigation receivers. A navigation receiver receives and processes radio signals transmitted by satellites located within a line-of-sight distance of the receivers. The satellite signals include carrier signals that are modulated by pseudo-random binary codes.
The receiver measures the time delay of the received signal relative to a local reference clock or oscillator. Code measurements enable the receiver to determine the so-called pseudo-ranges between the receiver and the satellites. The pseudo-ranges are different from the ranges (distances) between the receiver and the satellites due to various error sources and variations in the time scales of the satellites and receiver. If the number of satellites is large enough, then the measured pseudo-ranges can be processed to determine user's code coordinates and coordinate time scales. These receivers typically provide meter-level accuracy.
To improve the accuracy of these receivers beyond meter-level accuracy, differential navigation (DN) has been developed. In differential navigation, the task of finding the user position, also called a Rover, is performed relative to a Base station. The precise coordinates of the Base station are known because the Base station is stationary during measurements. The Base station has a navigation receiver which receives and processes the signals of the satellites. These signal measurements are transmitted to the Rover via a communication channel (e.g., wireless). The Rover uses these measurements received from the Base station, along with its own measurements taken with its own navigation receiver, in order to precisely determine its location. The determination of the Rover's location is improved in the differential navigation mode because Base station measurements are also used in the determination. Base station measurements can compensate for the major part of strongly correlated errors. A system that uses DN mode based on measuring pseudo-ranges only is called a Differential Global Positioning System (DGPS).
The accuracy of a location determination using differential navigation may be improved further by supplementing the pseudo-range measurements with measurements of the phases of the satellite carrier signals. If the carrier phase of the signal received from a satellite in the Base station receiver is measured and compared to the carrier phase of a signal from the same satellite measured in the Rover receiver, the accuracy of a measurement may be within several percent of the carrier's wavelength. Real-time carrier signal based differential navigation is often referred to as real-time kinematic (RTK) navigation. The practical implementation of these advantages often encounters the problem of ambiguity resolution for phase measurements. Some multi-frequency receivers can receive satellite signals within multiple frequency bands. As a result, these receivers may be able to measure carrier phases on multiple frequencies, thereby allowing for ionosphere delay corrections and facilitating ambiguity resolution.
In many instances, the Rover receiver operates in a complicated environment in which various external influences cause measurement errors. For example, external signals may interfere with the satellite signals, and structures and terrain may result in multipath errors. Errors are often classified as normal errors or abnormal errors. Normal errors are related to a receiver fundamental error source and relatively weak signals which are reflected from local objects. There are normal errors due to parasitic changes in wave propagation delay in atmosphere, inaccurate knowledge of satellite trajectories, onboard clock offset, thermal noise and other reasons. Abnormal errors are large errors which may prevent the system from calculating an accurate location. Such abnormal errors are occasionally a consequence of occasional spikes of intrinsic noise. More often, however, they are the result of exposure of the receiver to some type of interference. For example, strong reflected signals and extreme radio interference can cause abnormal errors. Short-term partial shading of the Rover receiver by a local object may also considerably increase measurement errors in one or more “shaded” satellite channels due to radio wave diffraction. If a satellite is completely shaded (i.e., blocked), tracking in the channel is interrupted, and the measured delay drifts because of reflected, still unshaded signals. Dynamic effects on the receiver (i.e., certain motion of the Rover) may also cause abnormal errors. Impulse accelerations or jerks impact both the receiving antenna and the quartz of the local reference oscillator and may result in the drift of the intermediate carrier frequency and measured phase.
One specific type of abnormal error is a cycle slip in phase lock loop (PLL) circuits of the receiver. The PLL circuits track the satellite carrier signal. After a cycle slip occurs, the PLL circuit locks to a new point of steady balance, after which it goes on with tracking the satellite carrier signal. As a result of the cycle slip, an abnormal error equal to several integer numbers of half-cycles is introduced into the full phase measurements. There are special techniques to detect cycle slips and to avoid errors propagating into final coordinates.
Much of the advancements in satellite positioning equipment has been directed to suppressing the various types of errors. There are techniques and devices that detect and eliminate measurements from some satellites when strong distortion is present. There are also techniques for analyzing observation results which allow for redundancy of a number of satellites to isolate a few unreliable measurements. Methods of reducing multipath are based on techniques for isolating and eliminating reflected signals that are delayed relative to the direct signal.
GNSS equipment may use receiver channels that are capable of receiving and co-processing signals from a plurality of satellites. This is advantageous for various reasons. First, the redundancy of satellite channels enables the use of statistical processing techniques (for instance, least squares techniques (LST)) in the determination of coordinates. The greater the number of satellites, the more efficient the process of averaging, and more accurate data may be obtained. Further, the more satellites, the more favorable their constellation for a specific user is, and better dilution of precision (DOP) can be gained. Finally, the redundancy of satellite channels enhances the ability to detect and eliminate abnormal measurements.
The European Union has launched the Galileo satellite radio navigation system. There remains a need to have a receiver that can efficiently receive and process the active satellite navigation systems (GPS (USA), GLONASS (Russia), and Galileo (European Union)).